1. Field of the Invention
This invention relates to a demagnetizing apparatus. In particular, this invention relates to a technology for erasing, using a recording or reproducing magnetic head, information recorded on concentric tracks on a magnetic recording medium, track by track. The demagnetizing apparatus according to the present invention erases wide-band, high-frequency recorded signals such as signals which are frequency-modulated (FM) with video signals. The apparatus according to the present invention especially eliminates the need for a relay to switch a single magnetic head to recording or reproducing.
2. Description of the Related Art
An AC erasing method has been well known as a method for erasing information recorded on a magnetic recording medium such as magnetic tapes or magnetic disks. This method applies an AC magnetic field to the magnetic recording medium to be erased so that the intensity of the magnetic field gradually decreases from the saturation level to lower levels. The gradually decreasing intensity of AC magnetic field causes the hysteresis loop of magnetization in the magnetic recording medium to gradually reduce, ultimately to zero. Thus, once saturated, the magnetic field gradually reduces, while depicting loops, down to the magnetic neutral point, thereby removing residual magnetism.
The AC demagnetization is normally accomplished using a bulk eraser (hereinafter referred to as "Method 1") or an erasing magnetic head (hereinafter referred to as "Method 2"). When using the Method 1 with the bulk eraser, it is operated so that the amplitude of the AC magnetic field generated by the eraser decreases gradually. When using the Method 2 with the erasing magnetic head, on the other hand, the polarity of the magnetic field generated by the magnetic head must alternate many times while a point of the magnetic recording medium is passing over the gap of the magnetic head and an amplitude of the magnetic field applied to the point decreases gradually. For the purpose, the gap of the erasing magnetic head is designed very wide, for example, as several tens of microns.
However, the Method 1 erases all the tracks of the magnetic recording medium and, therefore, cannot be used for erasing a particular track.
On the other hand, the Method 2 can be used for erasing track by track. However, a separate erasing head is required in addition to the recording or reproducing head, which leads to an increase in cost of the magnetic recording or reproducing system, or makes it difficult to design a compact unit. Moreover, recently developed electronic still cameras or video recording systems, in which video signals are recorded on concentric tracks of a small magnetic disk with a diameter of about 47 mm, have no space to contain the erasing magnetic head. Therefore, such an erasing magnetic head is not practically applicable.
It will be convenient if a desired track can be erased as with the Method 2 using a recording or reproducing magnetic head. This can be made with no major problems in systems for recording or reproducing signals of relatively low frequencies such as an audio tape recorder. However, since systems for recording or reproducing high-frequency signals, such as a magnetic disk-type electronic still camera, have a recording or reproducing magnetic video head with a gap width on the order of 0.1 micron, it is practically impossible to use AC demagnetization in such systems. For example, in an electronic still camera system using a 47 mm diameter magnetic disk rotating at 60 revolutions per second, which records video signals on concentric tracks on the magnetic disk, one field on one track, the recording or reproducing video head has a gap width of 0.25 to 0.35 microns so that signals with high frequencies up to about 10 MHz can be recorded. To accomplish erasing as the Method 2 using a video head with such a narrow gap, the video head must generate a magnetic field with an extremely higher frequency, for example, of 50 to 100 MHz, far exceeding the upper limit of the recording or reproducing frequencies and with a sufficiently high amplitude. However, it is not easy, with currently available technologies, to manufacture a recording or reproducing video head that generates such a high frequency magnetic field and, even if manufactured, it will be extremely expensive. Moreover, even if the high frequency magnetic field is generated, because of its high frequency, the field can reach only a very thin surface layer of the magnetic recording medium. As a result, from among the recorded video signals, low-frequency components of about 1 MHz in frequency recorded down to deep layers of the magnetic recording medium remain unerased. For the above described reasons, the AC demagnetization using a recording or reproducing video head has not been practically used.
There is another method for erasing track by track using a recording or reproducing magnetic head, in which a single-frequency signal with a relatively high frequency within the recording or reproducing frequency range is recorded overwritingly on a track to be erased. This method erases previously recorded signals in some measure. However, since the overwritten signal remains on the track, this method cannot provide a substantial erasing. Furthermore, this method is defective in that, when the previously recorded signals have a wide frequency range, such as video signals, a substantial level of low-frequency components remains unerased.
As described above, there have been various defects and limitations in the methods for erasing information on tracks by applying a high-frequency magnetic field using a recording or reproducing magnetic head. For a system in which a single magnetic head can repeatedly scan the same track, such as recording systems using magnetic disks, it has been known that easing is effectively made by applying DC current pulses, such as disclosed in Japanese Patent Publication No. 19378/1972 and No. 1409/1976, and Japanese Patent Application Laying-open No. 66414/1973. These disclosed methods use a plurality of square current pulses with alternating polarities and gradually decreasing levels. The polarity of the square current pulses changes at every turn of the magnetic disk. These erasing methods are expected to provide the same erasing effect as with the AC demagnetization, described in the method with the bulk eraser, while the magnetic disk is rotating at a normal speed, with no residual low-frequency components of previously recorded signals.
However, in an erasing method using DC current pulses, since in principle it is necessary to apply a DC current with alternating polarities to the magnetic head coil, an erasing signal circuit and a recording or reproducing signal circuit cannnot be switched at an input/output terminal for recording or reproducing signals, for example, the secondary side (amplifier side) of a step-up transformer, as in the AC demagnetization, but must be switched at the primary side of the step-up transformer where weak signals are involved. Use of a relay circuit for switching the erasing signal circuit and the recording or reproducing signal circuit leads to a complex circuit package which makes it difficult to design a compact unit. For the reasons, this method has not been used in a reliable, practical erasing system having a switching circuit for erasing and recording or reproduction.